This invention relates to the ophthalmic art and has particular relationship to the location of the critical center of each spectacle lens relative to the pupils of the patient. In the case of progressive-power lenses the critical center is the point where the progression begins. In the case of some progressive lenses the critical center is the optical center of the lens (Grolman, U.S. Pat. No. 4,055,900).
Spectacle lenses are formed from blanks whose surfaces are ground so that they have the required optical power. These blanks are usually circular and of substantially larger diameter than the largest dimension of a spectacle lens. Typically, a blank may be three inches in diameter. Each blank is provided with a mark defining the critical center point through which the visual axis of the eye should pass. This condition must be met to achieve an accurate centering of the spectacle lenses with respect to the pupils of the patient and their position in the frame. The visual axis is an imaginary line of sight which exits the ocular globe (eyeball) through the center of the pupil. The mark in the blank defines the point through which the visual axis should pass when the line-of-sight of the patient is in the normal primary straight-ahead position (as if both eyes are looking at infinity). The point of intersection is referred to as the point of intersection with the primary plane. The primary plane is a plane surface defined by the contour of the spectacle lens as it exists when mounted in the frame in normal use. For practical purposes, the plane surface may be a plane simulated lens.
In accommodating spectacles to the eyes of a patient, it is essential that after the spectacle lens is formed from the blank, the mark which was on the blank be on the visual axis when the line-of-sight is in the normal primary straight-ahead position. What this requires is that the height of the center of each pupil, in the primary straight-ahead position, above the point on the rim vertically below the center of the pupil must be determined accurately. This determination must be made separately for each eye as the pupils are not always symmetrical with respect to the tip of the nose or the center of the bridge of the nose or situated in the same horizontal plane.
In the case of a single-vision lens, this problem presents no difficulty. The intersection with the visual axis is typically at the optical center of the lens which is usually coincident with the geometric center. For bi-focal or tri-focal lenses which have demarcation lines between the lens sections of different power, the demarcation lines can be used as reference to orient the point of intersection of the lens by the visual axis. This practice has drawbacks, but it has served reasonably. Progressive power lenses have no demarcation lines and present a difficult problem. For progressive-power lenses it is important that the visual axes for both eyes be oriented at the optical threshold where the progressive power begins; i.e., where the power begins to change. As the eyes move together vertically in the progressive pathway, they should look through parts of the lenses of the same progressive power. This condition is governed by the choice of spectacle frame or contour of lenses. Once the frame or contour has been chosen, it must be insured that the respective dimensions of the frame or contour selected allow an accurate centering of the spectacle lenses with respect to the pupils of the patient. This requires that the visual axis pass through the critical center of the spectacle lens. It is an object of this invention to assure an accurate centering of spectacle lens with respect to the pupils of a patient particularly for progressive-power lenses, but also for multi-focal lenses in which there are demarcations between the sections of different power.
In accordance with the teachings of the prior art, the point where the visual axis should intersect the primary plane of progressive-power lenses is determined by manual measurements. The monocular pupilary distance is measured; this is the distance between the tip of the nose or the center of the nose bridge and the pupil in the primary straight-ahead position of the eyes of the patient. A frame or contour is then selected. The distance from the lowest point of the frame or of the periphery of each lens, to the center of the pupil of each eye is measured with the eyes in the straight-ahead position. A correction for distance sight is added. This data is supplied to the ophthalmic laboratory which produces the lenses with the critical center mark of the blank positioned so that visual axis in the primary straight-ahead position passes through this mark.
The above-described prior-art practice has the disadvantages that it is not sufficiently precise. To make the measurements it is necessary that the pupil of each eye be partially obstructed by the scale, or other targeting devices. This militates materially against precision. Another factor is the phenomenon of parallax. By viewing the scale at a slight angle, the reading of a measurement may depart materially from the actual magnitude. In addition, this procedure avails only abstract measurements. There is no concrete structure against which the practitioner or the patient can verify that the visual axis measurements are correct.
The dissatisfaction with the above-described prior-art practice is manifested by the multitude of patents on the same subject. This prior art involves complex and costly measuring mechanisms, most of which intersect the line of sight and obscure part of the pupil, in fact the very point that is to be located.
It is an object of this invention to overcome the deficiencies of the prior art and to provide a relatively simple and uncomplicated apparatus and method for determining accurately the point of a spectacle lens, through which the visual axis should pass, in whose use and practice unobstructed view of the pupils of the patient shall be available and the effect of parallax shall be minimized and that shall also avail a concrete structure enabling the practitioner or patient to verify the visual axis sights.